WW-I-9: Intensity of storm surges

The picture shows a storm surge hitting a coastal structure. The sea as a whole is very agitated.Click to enlarge
Storm surges also pose threats to infrastructures close to coasts.
Source: Photograph: © Wojciech Wrzesień / stock.adobe.com

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

WW-I-9: Intensity of storm surges

The development of storm surges on North and Baltic Sea coasts shows rising trends for the five levels examined. The exception is Kiel where no significant trend has been discerned.

The line graph shows the mean value of the annual highest mean tidal water over 19 years for Cuxhaven (North Sea) from 1900, for Travemünde (Baltic Sea) from 1853, for Kiel (Baltic Sea) from 1901, for Wittdün (North Sea) from 1936, for Saßnitz (Baltic Sea) from 1954 and for Borkum (North Sea) from 1963. All time series except for Kiel show a significantly increasing trend.
WW-I-9: Intensity of storm surges

The line graph shows the mean value of the annual highest mean tidal water over 19 years for Cuxhaven (North Sea) from 1900, for Travemünde (Baltic Sea) from 1853, for Kiel (Baltic Sea) from 1901, for Wittdün (North Sea) from 1936, for Saßnitz (Baltic Sea) from 1954 and for Borkum (North Sea) from 1963. All time series except for Kiel show a significantly increasing trend.

Source: BfG (level database of the Federal Water and Shipping Administration)
 

Increase in storm surges owing to sea level rise

When the water level on the North Sea coast is higher than 1.50 m above the average high-water level, this is referred to as storm surges. Storm surges occur when onshore wind categorised as storm or hurricane pushes large volumes of water towards the coast. If the resulting build-up of wind occurs during an astronomic high-water phase (such as MHW) on the North Sea coast, with a north-westerly wind force of Bft. 8-9 (on the Beaufort scale), this can lead to storm surges.

An additional increase in storm surges can result from external surges produced by wind fields in the North Atlantic which, similar to a tidal wave, traverse the North Sea in an anti-clockwise direction.

As a result of sea level rise in the North Sea in the course of the past one hundred years, storm surges start from a higher initial level and therefore cause waves to make landfall from a greater height. Apart from sea level rise, there is a problem, especially in estuaries, with increased storm-surge water levels owing to increased dyking and damming of the tributaries of Ems, Weser and Elbe. This means that the natural flood plains have been severely constricted.

In general, major storm surges entail damage to buildings and infrastructures near the coast. The North Sea coast in particular experienced major storm surges in the past. For almost 2000 years storm surges have been documented on German coasts. As early as 1219 the so-called ‘Grote Mandränke’ (the big drowning) caused the death of some 36,000 people. The storm surge of 16th February 1962 which affected the entire German Bight, and especially Hamburg, is deeply engrained in people’s memory. The big storm surge in northern Friesland in November 1981 caused extensive damage, in particular on North Sea islands which were unprotected by dunes or dykes. In December 1999 hurricane Anatol reached storm peaks up to 200 kilometres per hour and briefly caused very high increases in sea levels throughout the North Sea area. In December 2013 the entire North Sea area was affected by hurricane Xaver and storm surges which were quite severe in some places. The dykes on the mainland managed to withstand the onslaught of water masses, whereas on the islands in eastern and western Friesland, dunes were severely breached in many places.

As far as the Baltic Sea is concerned, the tides are not of major importance; here it is the duration and the intensity of the wind which determine whether storm surges occur. Water levels of one meter and more above the average water level are considered as storm surges here. The Baltic Sea is also known for the occurrence of ‘Seiches’ (French for standing wave) where water is pushed back from the German part of the Baltic Sea coast by westerly or north-westerly wind. Once the storm abates, the water than swashes landward thus leading to storm surges on the western coast of the Baltic Sea. As things progress, the waves of the Baltic Sea enter a pendulum motion (alternately swashing landward and seaward) until the motion subsides. The same phenomenon can be observed during prolonged easterlies. Furthermore, changes in air pressure also contribute to water level fluctuations by producing natural oscillation in water masses.

In the beginning of 2017 and 2019 severe storm surges occurred on the German coast of the Baltic Sea. In Wismar a sea level of 1.83 m above average was reached in the evening of 3rd January 2017 which was exceeded in the same place with a sea level of 1.91 m above average two years later on 2nd January 2019. After these events had taken place, damage was recorded especially on the coasts and beaches.

The increase in intensity of storm surges caused by rising sea levels is illustrated by the annual maximum High Water (HW) for the North Sea levels and for the annual maximum High Water measured for the Baltic Sea levels. The indicator reflects the highest (tidal) High Water per annum for the North Sea and the highest High Water per annum for the Baltic Sea. For these coefficients of water level, a ⁠moving average⁠ across 19 years was calculated by BfG.

In the case of selected North Sea levels a trend can be derived regarding the magnitude of sea level rise. Likewise, regarding the Baltic See coast a cyclical pattern and a periodicity of 30 to 40 years can be observed which overlies the trend. The illustration based on average values measured across 19 years has not revealed any extreme individual events. It is only accumulations of such events which lead to increasing values.

 

Interfaces

WW-I-8: Sea levels

WW-R-4: Investment in coastal protection

 

Objectives

Requirements of regional planning for the protection from increasing storm surges and floodwater risks (DAS, ch. 3.2.14)